The present invention relates generally to pattern recognition using either spatially coherent or spatially incoherent radiation. More particularly, the present invention relates to optical correlation utilizing electronic preprocessing of the pattern or image before it is input to the optical correlator.
The desirability of a relatively simple, reliable apparatus for detecting and recognizing specific patterns has long been recognized. For example, a device which would automatically determine whether patterns such as airplane images are included on aerial photographs would reduce the time and error of visual examination of photographs. As another example, a device which would automatically recognize and distinguish between different physical objects such as tools would be of great value in industrial robots.
One type of pattern recognition system, described in "Signal Detection By Complex Spatial Filtering," by A. Vander Lugt, IEEE Transactions On Information Theory, Vol. IT-10, p. 139, April, 1964, includes a hologram recording of a Fourier transformed image of the pattern to be recognized. Generally speaking, a hologram is a recording of both amplitute and phase of a light wave front. A Fourier transformed hologram is a recording of the amplitude and phase of the Fourier transform of the light waves reflected from an object.
The Vander Lugt apparatus requires the use of spatially coherent light, such as laser light. In order to process images in the Vander Lugt system, graphic information must be displayed on a spatial light modulator and illuminated by the laser light in order to provide an optical input signal which is spatially coherent.
The use of a hologram for pattern detection and recognition by correlation filtering using spatially incoherent light was suggested in 1965 by Armitage and Lohmann in Appl. Opts. 4, 461 (1965). The processing of images or two dimensional functions with spatially incoherent radiation has several distinct advantages over systems utilizing coherent radiation. Since coherent radiation is unnecessary, the need for a laser is eliminated. In addition, there is no need for a spatial light modulator to modulate the laser radiation. Spatial light modulators can be extremely expensive. Moreover, the positioning of the matched spatial filter in the filter plane is not critical in an incoherent system as it is in a system which uses coherent radiation.
The system described by Armitage and Lohmann included an incoherent light source, lenses and a matched spatial filter in the form of a Fourier transformed hologram. The Armitage and Lohmann system is insensitive to the location of the hologram along any axis and as long as the hologram is not rotated in the x-y plane. This is an improvement over the coherent system, wherein the relative position of the hologram is much more critical. In the incoherent system, pattern detection is achieved by the use of a matched filter initially formed from an image of the pattern to be detected. Like the coherent system, the incoherent system is "shift invariant", that is, because the matched filter is a Fourier transformed hologram, the system is able to recognize a pattern independently of its location with respect to the x-y axes.
Since the early work of Vander Lugt and Armitage and Lohmann, both coherent and incoherent spatial filtering have been widely studied. Heretofore, however, incoherent optical correlation has been less sensitive than coherent optical correlation. That is, an incoherent system's ability to discriminate among several objects is inferior to that of a coherent system.
The present invention is applicable to both coherent and incoherent optical correlation and recognizes that electronic preprocessing of the image which forms the input to the optical correlator may be performed to improve the correlation of the pattern to be recognized over an unpreprocessed image. By electronically preprocessing the image, an optical correlator may correlate to specific characteristics and features of the image which could not be done from the unpreprocessed image. When electronic preprocessing of an image is used in conjunction with incoherent optical correlation, performance and accuracy of the correlation is improved. Heretofore, no one has recognized the advantages to be obtained by this approach.